1. Field of the Invention
The present invention relates to apparatus and method for manufacturing a glass-base-material. More particularly, the present invention relates to apparatus and method for manufacturing a glass-base-material that measures the weight of glass soot accumulated on the glass rod.
2. Description of the Related Art
FIG. 1 shows an apparatus for manufacturing a glass-base-material, which is a base material of an optical fiber, using outside vapor deposition (OVD) method. The glass-base-material manufacturing apparatus has a chamber 1, a burner 2, bases 3, a rotation unit 4, measuring unit 7, chucks 8, bearings 9, and a drive shaft 13. The glass-base-material manufacturing apparatus is installed on a common base 10. The chamber 1 accommodates the burner 2, the bases 3, the rotation unit 4, the chucks 8, the bearings 9, and the drive shaft 13.
The chucks 8 hold both ends of the glass rod 5. The drive shaft 13 connects the chuck 8 and the rotation unit 4 to transfer the movement of the rotation unit 4 to the chuck 8. The rotation unit 4 rotates each end of the glass rod 5 by rotating the chucks 8 via the drive shaft 13. The bearing 9 is provided between the chuck 8 and the rotation unit 4 to support the drive shaft 13.
The burner 2 hydrolyzes a raw material gas, which is a base material of a glass-base-material, into glass soot and accumulates the glass soot around the outside surface of the glass rod 5, which is rotated by the rotation unit 4, to form a glass-base-material 150. The burner 2 also moves along the longitudinal direction of the glass rod 5 to accumulate the glass soot evenly along the longitudinal direction of the glass rod 5. After the glass-base-material 150 is manufactured, the glass-base-material 150 is sintered in the heat furnace to be dehydrated and vitrified to become a preform.
If the glass soot is accumulated unevenly around the glass rod 5, the cross section of the preform, which is made by sintering this glass-base-material 150, does not have an accurate circle shape. Also, the core/clad ratio becomes inconstant along the longitudinal direction of the glass rod 5. Therefore, it is important to control the rate of increase in the weight of glass-base-material 150. To control the rate of increase in the weight of glass-base-material 150, it is important to measure the weight of glass-base-material 150 accurately.
Therefore, conventionally, the total weight of the chamber 1 that includes the weight of the bases 3, the rotation units 4, the chucks 8, and the bearings 9 are measured by the measuring unit 7. Then, the weight of the glass-base-material 150 is measured by deducting the tare weight, which is the initial weight measured before the beginning of the accumulation process of the glass soot, from the measured total weight.
It was difficult to measure the weight of the glass-base-material 150 accurately by the conventional method because the size of the measuring unit has to be large enough to measure the total weight of the glass-base-material manufacturing apparatus including the weight of the chamber 1.
FIG. 2 shows an apparatus described in Japanese Patent Application Laying-Open No. 2000-86271. The apparatus shown in FIG. 2 has a bearing case 11, which accommodates a plurality of bearings 9A that support the drive shaft 13A. A measuring unit 7 is provided under the bearing case 11 to support the bearing case 11. The measuring unit 7 measures the weight of the glass-base-material 150 by measuring the weight of the bearing case 11. A revolute joint 12 joins the drive shaft 13A supported by the bearings 9A and the drive shaft 13B supported by the bearing 9B.
During the accumulation process of the glass soot, the glass rod 5 may expand or deflect along the axial direction because of the heat applied on the glass rod 5. Therefore, the moment of the rotation of the glass rod 5 around the axis of the glass rod 5 becomes uneven and causes friction on the revolt joint 12 which influences the accuracy of measuring the weight of the glass-base-material 150. Thus, the apparatus shown in FIG. 2 could not accurately measure the weight of the glass-base-material 150.